Magnetic resonance imaging device

ABSTRACT

In a contrast MRA measurement, sampling order of k-space is controlled considering the distance from the origin such that sampling the low-frequency data is performed a time when the contrast concentration reaches it peak. First, the sampling points of k-space are divided into two groups. Then, a measurement of the first group is started a time when the contrast concentration of a blood vessel of interest becomes high and is controlled from the high-frequency component to the low-frequency component such that the distance of a sampling point from the origin progressively decreases. A measurement of the other group, which is performed successively, is controlled from the low-frequency component to the high-frequency component such that the distance of a sampling point from the origin progressively increases. According to this ordering, influence of measurement time error in the contrast MRA measurement can be reduced and the whole blood vessel can be imaged with high contrast. In addition, an artery can be selectively imaged.

FIELD OF THE INVENTION

The present invention relates to a magnetic resonance imaging apparatus(MRI apparatus) for obtaining tomograms of desired portions of an objectto be examined using nuclear magnetic resonance (NMR). In particular, itrelate to an MRI apparatus capable of obtaining desired range of imagesof excellent quality in minimal time to enable visualization of movementin the vascular system.

BACKGROUND OF THE INVENTION

An MRI apparatus utilizes NMR to measure density distribution andrelaxation time etc. of atomic nuclear spins (referred as spinshereinafter) in a desired portion of an object to be examined anddisplays images of desired slices of the object produced from themeasured data. Conventional MRI apparatuses have a blood flow imagingfunction called MR angiography (MRA). This function includes a methodusing a contrast agent and a method using no contrast agent.

In a common method using a contrast agent, a gradient echo type sequenceof short TR (repetition time) is used in combination with aT1-shortening type contrast agent such as Gd-DTPA. The method utilizesthe fact that blood spins containing T1 shortening contrast agent arenot likely to be saturated by the repeated excitation of a short TRbecause the blood spins have a shorter T1 than those of surroundingtissues and generate high-strength signals relative to the surroundingtissues, and enables to visualize blood vessels filled with bloodcontaining a contrast agent with high contrast relative to the othertissues. Measurement of volume data including blood vessels, typicallythree-dimensional measurement, is conducted while the contrast agentremains in the blood of interest and the obtained three-dimensionalimages are combined and subjected to projection process to depict bloodflow. In order to obtain information of wide range and high resolution,sequence based on the three-dimensional gradient echo method forobtaining three-dimensional data is generally employed.

In order to produce good images in such a three-dimensional contrastMRA, the following factors are important. (1) A manner of injecting acontrast agent, and (2) Measurement time or timing. With regard to (1),the contrast agent should be injected such that its high concentrationis maintained in a blood vessel of interest in a stable manner. For thispurpose, a rapid injection method using an automatic injection machineis generally utilized.

With regard to (2), if arteries are selectively imaged, for example, theimaging timing is determined such that the concentration of the contrastagent becomes high when data are acquired. Ideally, the concentration ofthe contrast agent reaches its peak value when a central part (lowfrequency region) of the k-space which controls the image contrast issampled. The timing is determined corresponding to date-acquisitionordering of an employed pulse sequence.

Conventional data-acquisition ordering includes a sequential ordering inwhich data are acquired from one high-frequency side of k-space towardthe other high-frequency side via a low-frequency region, and a centricordering in which data are acquired from low-frequency region of thek-space toward both high-frequency sides alternately. Generally, thecentric ordering is employed. In a centric ordering in athree-dimensional measurement, one of a phase encoding loop and a sliceencoding loop is set to be an outer loop and the other to be an innerloop, and either or both is controlled by a centric ordering.

However, as shown in FIG. 1(b), the centric ordering in this case is nota centric ordering in a true sense because a distance between thek-space origin and a sampling point fluctuates, and therefore likely tobe influenced by movement of an examined object and makes separation ofarteries and veins insufficient.

For solving this problem, an elliptical centric ordering is proposed, inwhich sampling order is controlled in view of the relative FOV (Field ofView) such that the distance from the ky-kz space origin to a samplingpoint increases as the measurement proceeds (FIG. 1(c)) (“Performance ofan Elliptical Centric View Order for Signal Enhancement and MotionArtifact Suppression in Breath-hold Three-Dimensional Gradient EchoImaging. Alan, et al. Magnetic Resonance in Medicine 38:793-802,1997”)

This data-acquisition method enables to produce arterial imagesselectively by starting measurement at the time when the concentrationof a contrast agent in the blood vessel of interest increases sincelow-frequency region data that dominates the image contrast are measuredat the beginning of the measurement time.

Although the above-mentioned centric ordering and elliptical centricordering enable to determine the image contrast in the early stage ofmeasurement and are efficient for obtaining arterial images, if theoptimal measuring moment is missed, the low-frequency information isacquired during the concentration of a contrast agent is low and therebyimage quality becomes degraded. Especially, if the measuring time is tooearly, the low-frequency data is sampled in the time period when signalsof a blood vessel are extremely low and the high-frequency region datais sampled in the time period when the signals of the blood vessel arehigh. This causes rinsing artifacts having no direct current component.In addition, the measurement time is prolonged because overallmeasurement is performed from the origin as a center toward thehigh-frequency region of the k-space horizontally or vertically.

On the other hand, the sequential ordering enables to produce stableimages in which remarkable artifact is not likely to generate even ifthe measurement timing is somewhat wrong. However, this ordering issusceptible to movement of an examined object similarly to theaforementioned centric ordering and separation of artery and veinsbecomes insufficient.

Accordingly, an object of the present invention is to provide an MRIapparatus capable of visualizing a whole of a blood vessel of interestwith high contrast in minimal time while reducing the influence of timeshift (error) from an optimal measurement time. Another object of thepresent invention is to provide an MRI apparatus which is insusceptibleto the influence of movement of an examined object and capable ofvisualizing arteries and veins separately. Yet another object of thepresent invention is to provide a data-acquisition method suitable forMRA.

DISCLOSURE OF THE INVENTION

In order to achieve the above-mentioned objects, an MRI apparatus of thepresent invention employs a data-acquisition method in which samplingpoints of k-space are divided into two groups and, in the first groupwhich is measured first, sampling order is controlled from thehigh-frequency region toward the low-frequency region such that thedistance from the k-space origin to a sampling point progressivelydecreases and, in the second group, sampling order is controlled in anopposite manner from the low-frequency region toward the high-frequencyregion such that the distance from the k-space origin to a samplingpoint progressively increases.

Specifically, an MRI apparatus of the present invention comprises staticmagnetic field generating means for generating a static magnetic fieldin a space where an object to be examined is placed, gradient magneticfield generating means for applying gradient magnetic fields in theslice direction, phase encoding direction and readout direction,transmitting means for applying high-frequency magnetic field to causenuclear magnetic resonance in atomic nuclei of a living tissue of theobject, receiving means for detecting echo signals emitted by thenuclear magnetic resonance, control means for controlling the magneticfield generating means, transmitting means and receiving means, signalprocessing means for performing image reconstruction operation using theecho signals detected by the receiving means, display means fordisplaying the produced image, wherein the control means performs athree-dimensional sequence including a slice encoding step and a phaseencoding step, upon performing the sequence, divides sampling points ofa k-space defined by a slice encode number and phase encode number intotwo groups and controls the gradient magnetic field generation means ofslice direction and phase direction such that the distance from theorigin of the k-space to a sampling point progressively decreases in themeasurement of the first group and the distance from the origin of thek-space to a sampling point progressively increases in the measurementof the second group.

The manner of dividing the sampling points of the k-space into two groupmay be such that at least one of the groups includes sampling pointsfrom a low-frequency region to a high-frequency region and the otherincludes at least sampling points of a low-frequency region. A number ofsampling points which are really measured may be the same or differentin the two groups. Namely, either of the two groups may includenon-sampling points (the points which are not measured).

Specifically, one embodiment is that sampling points are divided intotwo groups depending on the region of the k-space. According to thisembodiment, the control system performs a three-dimensional sequenceincluding a slice encoding step and a phase encoding step, uponperforming the sequence, divides the k-space defined by the slice encodenumber and the phase encode number into two regions, and controlsgradient magnetic field generating means of the slice direction and thephase direction such that, in one of the regions, the distance from theorigin of the k-space to a sampling point progressively decreases and,in the other region, the distance from the origin of the k-space to asampling point progressively increases.

In another embodiment, the sampling points of the k-space are dividedinto two groups which are in a relation of complex conjugate. Accordingto this embodiment, the control system performs a three-dimensionalsequence including a slice encoding step and a phase encoding step, uponperforming the sequence, divides sampling points of the k-space definedby a slice encode number and phase encode number into two groups whichshare the origin and are in a relation of complex conjugate, andcontrols gradient magnetic field generating means of the slice directionand phase direction so that, in one of the groups, the distance from theorigin of the k-space to a sampling point progressively decreases and,in the other group, the distance from the origin of the k-space to asampling point progressively increases.

In this embodiment, it is preferable that adjacent two sampling pointsbelong to different groups. In order to satisfy the condition thatsampling points of the two groups are complex conjugate each other, someof the adjacent sampling points near the origin are required to belongthe same group. Accordingly, in the specification, the wording “todivide such that the adjacent sampling points belong to the differentgroups” means the state satisfying as much as possible the conditionthat the two groups are in a relation of complex conjugate and theadjacent sampling points belong to the different groups.

In yet another embodiment of the MRI apparatus of the present invention,the control system does not measure all of the sampling points in one ofthe two divided regions but measures a smaller number of sampling pointsthan that of the other region. Or the control system does not measureall of the sampling points in one of the two groups but measures asmaller number of sampling points than that of the other group.

A three-dimensional image data-acquisition method of the presentinvention comprises, when a three-dimensional image data is acquired byrepeating a plurality of times a step comprising selective excitation ofa predetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions, andcollection of echo signals generated from the region while changing theintensities of the gradient magnetic fields,

divides a measurement space defined by the gradient magnetic fieldsencoding in the two directions, and

performs measurement of the tow divided regions sequentially such thatthe distance from the origin of the measurement space to a samplingpoint gradually decreases in the measurement of a region which ismeasured first and the distance from the origin of the measurement spaceto a sampling point gradually increases in the measurement of the regionwhich is measured thereafter.

In the data-acquisition method of the present invention, when there areseveral sampling points whose distances from the origin are the same,the nearest sampling point from the latest measured point in the k-spaceis preferably measured next.

A three-dimensional image data-acquisition method of the presentinvention, when three-dimensional image data is acquired by repeating aplurality of times a step comprising selective excitation of apredetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions, andcollection of echo signals emitted from the region while changing theintensity of the gradient magnetic fields,

divides sampling points on a measurement space defined by theintensities of the encoding gradient magnetic fields of the twodirections into two groups such that the first group and the secondgroup share the origin and that adjacent and conjugating sampling pointsbelong to different groups, and

performs measurement of the first group and the second group in thisorder such that, in a measurement of the first group, the distance of asampling point from the origin of the measurement space progressivelydecreases and, in a measurement of the second group, the distance of asampling point from the origin of the measurement space progressivelyincreases.

According to the data-acquisition method of the present invention, asshown in FIG. 1(a), measurement can be performed without fluctuation ofdistance from the origin and a blood vessel concerned can be visualizedwith high contrast by making the time of measuring the lowest frequencycomponent coincide with the time when the contrast agent enhance thesignal intensity of the blood vessel most. In addition, even the time ofmeasuring the low-frequency component is somewhat shifted from the peakof the signal intensity, proper sampling of the low-frequency componentcan be assured and images are not degraded.

For reference, variations of distance from the origin of the k-space inthe conventional centric ordering, elliptical centric ordering andsequential ordering are shown in FIG. 1(b)-(d).

According to a preferable embodiment of the data-acquisition method ofthe present invention, a part of all of the sampling points is measuredin the first group and all of the sampling points are measured in thesecond group.

Since the two groups are in a relation of complex conjugate, even if apart of one group is not measured, data which are not measured can beestimated from data of the other group. Particularly when a measurementof the first group is performed during the density of the contrast agentis increasing to its peak, sampling of unnecessary data having lowsignal intensity is avoided and thereby images of good quality can beobtained.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 shows exemplary views of a data-acquisition method employed bythe MRI apparatus of the present invention and conventionaldata-acquisition methods.

FIG. 2 is an overall block diagram of the MRI apparatus of the presentinvention.

FIG. 3 shows an example of a pulse sequence of contrast MRA measurementperformed by the MRI apparatus of the present invention.

FIG. 4 shows an example of a k-space data-acquisition method accordingto the present invention.

FIG. 5 is an exemplary view of MRA measurement according to the MRIapparatus of the present invention.

FIG. 6 shows simulation results of MRA measurement according to the MRIapparatus of the present invention and the conventional MRA measurement.

FIG. 7 shows another example of a k-space data-acquisition orderingaccording to the present invention.

FIG. 8 shows yet another example of a k-space data-acquisition orderingaccording to the present invention.

FIG. 9 shows an example of a k-space data-acquisition ordering accordingto the present invention.

FIG. 10 shows an exemplary view of MRA measurement according to the MRIapparatus of the present invention.

FIG. 11 shows yet another example of a k-space data-acquisition orderingaccording to the present invention.

FIG. 12 shows an exemplary view of an image reconstruction method usingthe data-acquisition method of FIG. 10.

FIG. 13 shows a simulation result for evaluating the MRA measurement bythe MRI apparatus of the present invention.

FIG. 14 shows simulation results of MRA measurement according to the MRIapparatus of the present invention and the conventional MRA measurement.

FIG. 15 shows another example of a k-space data-acquisition orderingaccording to the present invention.

THE BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

An embodiment of the present invention will be explained with referenceto the attached drawings hereinafter.

FIG. 2 is an overall block diagram of the MRI apparatus of the presentinvention. The MRI apparatus is for obtaining tomograms utilizing NMRand comprises a static magnetic field generating magnet 2, a gradientmagnetic field generating unit 3, a sequencer 4, a transmitting unit 5,a receiving unit 6, a signal processing unit 7, and a central processingunit (CPU) 8.

The static magnet field generating magnet 2 generates a uniform staticmagnetic field around an object to be examined in a direction parallelor perpendicular to the body axis of the object and comprises means forgenerating a static magnetic field in a space around the object such asa permanent magnet, a resistive magnet or a superconductive magnet.

The gradient magnet field generating unit 3 comprises gradient magneticfield coils 9 wound in the direction of three axes, X, Y, Z, and agradient magnetic field power supply 10 for driving the gradientmagnetic field coils. The gradient magnetic field power supply 10 isdriven according to instructions from the sequencer 4 and appliesgradient magnetic fields Gx, Gy and Gz in the direction of the threeaxes, X, Y and Z, to the object. A manner of applying these gradientmagnetic fields determines a slice or slab of the object to beselectively excited and defines position or sampling order of samplingpoints in a measurement space (k-space).

The sequencer 4 operates under the control of the CPU 8, and sendsnecessary instruction for collecting data for obtaining tomograms of theobject to the gradient magnet field generating unit 3, the transmittingunit 5 and the receiving unit 6. The operation timing of the gradientmagnet field generating unit 3, the transmitting unit 5 and thereceiving unit 6 controlled by the sequencer 4 is called a pulsesequence. Here, a sequence for imaging three-dimensional blood flow isemployed as one of sequences. The control of the sequencer 4 will beexplained in more detail later.

The transmitting unit 5 is for producing an FR magnetic field in orderto cause nuclear magnetic resonance of nuclei of atoms constituting theliving tissues of the object in accordance with the RF pulse transmittedfrom the sequencer 4, and comprises an FR oscillator 11, a modulator 12,an RF amplifier 13 and an RF coil for transmission 14 a. Thetransmitting unit 5 amplitude-modulates the RF pulses output from the RFoscillator 11 by the modulator 12 in accordance with instructions fromthe sequencer 4. The amplitude-modulated RF pulses are amplified by theRF amplifier 13 and supplied to the RF coil 14 a located in the vicinityof the object 1 so that electromagnetic waves are radiated onto theobject 1.

The receiving unit 6 is for detecting echo signals (NMR signals)elicited through nuclear magnetic resonance of atomic nuclei of theliving tissues of the object 1, and comprises an RF coil 14 b forreceiving electromagnetic waves, an amplifier 15, a quadrature phasedetector 16 and an A/D converter 17. The electromagnetic waves of theobject responding to the electromagnetic waves radiated by the RF coil14 a of the transmitting unit is detected by the RF coil 14 b located inthe vicinity of the object 1. The detected echo signals are input intothe A/D converter 17 via the amplifier 15 and the quadrature phasedetector 16, converted into digital signals, which are sampled by thequadrature phase detector 16 with a timing instructed by the sequencer 4to become two-series data, and transferred to the signal processing unit7.

The signal processing unit 7 comprises the CPU 8, recording media suchas a magnetic disk 18 and magnetic tape 19, and a display unit 20 suchas a CRT. The CPU 8 performs operations onto echo signals (digitalsignals) input from the receiving unit 6 such as Fourier transform,calculation of a correction coefficient and image reconstruction, andthereby produces and displays a signal intensity distribution of acertain section or images produced by performing suitable arithmeticoperations on a plurality of signals as a tomogram on the display unit20. In FIG. 2, the RF coils 14 a, 14 b for transmission and receivingand the gradient magnetic field coils 9 are disposed within a magneticfield space of the static magnet field generating magnet 2 placed in aspace around the object 1.

A first embodiment of a blood flow imaging performed by the MRIapparatus according to the present invention will now be explained.

As mentioned previously, the sequencer 4 controls operation timings ofthe gradient magnet field generating unit 3, transmitting unit 5 andreceiving unit 6 according to the predetermined pulse sequence, here athree-dimensional MRA sequence. The pulse sequence, similarly to othersequences, is stored as a program in a memory provided in CPU8 andselected properly by a user corresponding to the purpose of imaging tobe performed. Specifically, if MRA measurement using a contrast agent isselected through an input module of CPU 8, the CPU 8 controls thesequencer 4 to perform the three-dimensional MRA sequence.

This pulse sequence is, as shown in FIG. 3 for example, a sequence basedon the gradient echo method and is common in the conventionalthree-dimensional MRA sequence. Specifically, after a region (slab)including a blood vessel of interest is excited by applying a selectinggradient magnetic field Gs together with an RF pulse RF, a gradientmagnetic field pulse Ge1 in the slice direction and a gradient magneticfield pulse Ge2 in the phase encode direction are applied and then areadout gradient magnetic field Gr is applied while reversing itspolarity to measure echo signals. Procedures from applying the RF pulseRF to measurement of echo signals are repeated while changing theintensities of the gradient magnetic field Ge1 in the slice directionand the gradient magnetic field Ge2 of the phase encode direction with apredetermined repetition time TR to acquire three-dimensional data.

The encode number of the slice direction and that of the phase encodedirection define an image resolution in the both directions and arepredetermined in consideration of a measurement time or the like. Forexample, the encode number of the phase encode direction is set to be128, 256, etc. and that of the slice direction is 10-30. The encodenumbers of the slice direction and phase encode direction also definek-space (provided that the slice direction is z and phase encodedirection is y, ky-kz space). In the sequence shown in FIG. 3, a signalmeasured when the intensity of the slice direction gradient magneticfield Ge1 has a value of Gz and the intensity of the phase-encodedirection gradient magnetic field Ge2 has a value of Gy is disposed at apoint (ky,kz) of the k-space corresponding to Gy and Gz.

Although the three-dimensional MRA sequence exemplified in FIG. 3 iscommon in the conventional MRA, data-acquisition method of this sequenceemployed in this embodiment is different from the conventional centricordering or elliptical ordering. In this method, the ky-kz space isdivided into two along with a ky-axis or kz-axis and, in one regionwhich is measured first, a sampling order is controlled from thehigh-frequency component toward the low-frequency component such that apoint whose distance from the origin is large is sampled first and apoint to be sampled becomes closer to the origin 0. Contrarily, in theother region, a sampling order is controlled from the low-frequencycomponent toward the high-frequency component such that the origin 0 orthe vicinity thereof is sampled first and a point to be sampled becomesmore distant from the origin 0.

In this case, if there are several points whose distances from theorigin are same, the nearest point from the last sampled point in thek-space is sampled next.

Namely, this data-acquisition method is defined by “AND” of twoconditions: 1) In one of the two divided regions, a measurement isstarted with the most distant point from the origin and then points tobe sampled are successively determined such that the distance from theorigin decreases and, in the other region, a measurement is started withthe origin or the vicinity of the origin and then points to be sampledare successively determined such that the distance from the originincreases. 2) Distance between the temporarily adjacent sampling pointsbecomes minimum.

Although the condition 2) is not indispensable in the data-acquisitionmethod of the present invention, when the spatial distance between thetemporarily adjacent sampling points is made as close as possible,artifact is suppressed. For this purpose, instead of satisfying theabove condition of a distance between two sampling points, a samplingpoint having the same ky value or the nearest ky value may be selectedas a next sampling point. Further, a sampling point may be determined inconsideration of not only relationship of the two sampling points butalso relationship among a plurality of sampling points which are samplednext or afterward.

As a simplified example of the afore-mentioned data-acquisition method,FIG. 4 shows a data-acquisition ordering of the k-space having 5*9matrix where the slice encode number is 5 and the phase encode number is9. In the figure, circled numbers indicate data-collection order. Inthis example, the k-space is divided into two by kz axis and, in thelower region (E→C region), data are sampled from a point (number 1)marked “start” in a high-frequency region toward the origin (number 25)in numerical order and, in the upper region (C→E region), data aresampled from the origin toward the high-frequency region and ended at anumber 45. The distance Δz between the adjacent points in the k-spacecoordinate is 1/FOVz.

Next, an embodiment of a contrast MRA measurement performed by theaforementioned MRI apparatus employing such a data-acquisition methodwill be explained with reference to FIG. 5.

First, an object to be examined is placed in a measurement space withinthe static magnet field magnet and an imaging region including a bloodvessel of interest is determined. Then, a timing measurement isperformed by a test injection method. In this method, a small amount ofa contrast agent (about 1-2 ml) is injected as a test dose in order toobtain a time-signal curve of the object portion as shown in FIG. 5.Arriving time t1 of the contrast agent is found from this curve and atiming of the measurement is determined based on the result. Instead ofthe test injection method, a method where an ROI (Region of Interest) isset in a specific portion within a monitored region, variation of signalintensity of that portion is monitored and a measurement is startedautomatically at a time when the signal intensity exceeds apredetermined threshold value, or a method where a blood vessel ofinterest is monitored in real time by repeating a short time measurementand display called fluoroscopy, and a measurement is started at a timewhen the signal intensity is properly enhanced can be employed for thetiming measurement. However, the test injection method is preferredbecause it enables to determine measurement timing accurately by using acontrast agent prior to a main measurement.

After the timing measurement, a main measurement is performed as shownin FIG. 5(b). The main measurement may be performed only after injectionof the contrast agent but preferably performed before and afterinjection of the contrast agent. The measurements are carried outsuccessively before and after injection for the same slice or slabposition under the same condition.

The imaging sequence is a short TR sequence based on thethree-dimensional gradient echo method as shown in FIG. 3. In this case,since an imaging object is blood flow, a gradient magnetic field forrephasing a dephase caused by flow, i.e., gradient moment nulling may beadded to the sequence. However, this is not essential and a simplegradient echo is rather preferable in order to shorten the TR/TE.

Once the repetition time TR of the pulse sequence and a matrix size(slice encode number and phase encode number) are determined, themeasurement time T is determined. Then starting time t2 (a time frominjection of a contrast agent to the beginning of measurement) isdetermined based on the arriving time t1 of the target blood vesselobtained in the aforementioned timing measurement such that data in alow-frequency region of the ky-kz space are sampled at a time when thecontrast agent arrives at the blood vessel.

First, a measurement of the E→C region shown in FIG. 4 is performed andthen a measurement of the C→E region is carried out. In this case, thesequencer 4 controls both a slice direction gradient magnetic fieldpulse and a phase encode direction gradient magnetic field pulse so asto sample from a high-frequency component toward a low-frequencycomponent sequentially in a region measured first (for example E→Cregion), and sample from a low-frequency component toward ahigh-frequency component sequentially in the region measured thereafter(for example C→E region). In this control, as mentioned previously, adistance from the origin to a sampling point gradually decreases in thefirst region and increases in the second region.

According to this control, the low-frequency component of the k-space issampled at a time when the contrast agent arrives at the target bloodvessel and the signal intensity of the blood becomes highest and thusimages of the artery can be visualized with high contrast. Since thereremains a time for sampling a low-frequency component on both sides ofthe arriving time t1 of the contrast agent as shown in FIG. 5(b), evenif the arriving time in the main measurement slightly differs from that(t1) determined in the timing measurement due to a slight change ofconditions between the two measurement, high quality images can beobtained without degrading the quality. Results of simulation ofseparation of artery-vein using different data-acquisition methods areshown in FIG. 6. The stimulation was conducted under condition of FOV:320, TF: 10 ms, phase encode number: 160, slice encode number: 16, imagematrix: 256*256, and slice thickness: 5 mm. Separation of artery-vein isexpressed using a ratio of signal intensity of an artery and a vein.

As understood from the figure, a signal ratio of the artery and vein islarger in the data-acquisition method of the present invention than inthe sequential ordering and elliptical centric ordering. Accordingly,even if there is an indistinguishable vein near an artery of interest,only the artery can be depicted with high contrast.

After three-dimensional image data are thus acquired before and afterinjection of the contrast agent, the image data are subtracted toproduce a three-dimensional data of only blood vessels. The subtractionmay be a complex subtraction performed between slices on the same sliceposition of the three-dimensional data. Subtraction of absolute valuesmay be employed. Such a method of deleting tissues other than bloodvessels using subtraction between images obtained before and afterinjection of contrast agent is a known method called 3D MR-DSA(DigitalSubtraction Angiography) and is not essential for the present invention.However, use of this method is preferable for imaging fine bloodvessels, which are otherwise difficult to be visualized withsufficiently high contrast to other tissues.

After the subtraction, the three-dimensional data may be projected in anarbitrarily direction such as coronal direction, saggital direction,transversal direction or the like to be observed in three dimensions. Aknown method of projection such as Maximum Intensity Projection may beemployed.

The first embodiment of the present invention has been explained usingthe examples shown in FIGS. 4 and 5. Various modifications thereof canbe employed. For example, although a sequence according to the gradientecho method is exemplified as a three-dimensional MRA sequence in theaforementioned examples, an EPI (Echo Planer Imaging) in which aplurality number of echo signals are measured at a single excitation ora multi-shot type EPI can be employed.

The k-space is divided by the kz axis in the aforementioned example butmay be divided by the ky axis. Further, although the case that data areacquired symmetrically in the divided regions has been explained, theregions to be sampled may be asymmetrical.

An example of the asymmetrical acquisition case will be explained withreference to FIG. 7. In this case too, k-space having a matrix size of5*9 and divided by the kz axis is exemplified. In the asymmetric dataacquisition, a measurement is started not from the high-frequency regionbut from the middle frequency region (number 1) in the first region,e.g., E-C region, toward low-frequency region (number 15) in numericalorder. After that, the overall C→E region is sampled from low-frequencyregion to high-frequency region similarly to the case shown in FIG. 4.Alternatively, an overall region measured first is sampled and thelatter half region is sampled from low-frequency region tomiddle-frequency region. Referring to the example shown in FIG. 7, ameasurement is started with a number 35 and performed to a number 1 indecreasing numerical order.

By making the number of sampling points in one of the two regions lessthan in the other region, a total measurement time can be shortened.Specifically, as shown in FIG. 5(c) and (d), by reducing the number ofsampling points in a region to be measured first or later, themeasurement time becomes shorter than the case shown in FIG. 5(b).Further, a measurement method shown in FIG. 5(d) is effective in a casethat the distance between a blood vessel of interest and a neighboringvein is short, and interval between an arrival time of the contrastagent at the blood vessel of interest and an arrival time at theneighboring vein is short. In this case, when the posterior region issampled from the low-frequency component toward the middle-frequencycomponent, signals from the neighboring vein is kept from getting mixed(a dot line signal intensity curve).

In this case, data which have not been sampled of the region having asmaller number of sampling points (data of the region shown by obliquelines) may be estimated from data of the other region which have beensampled, or may be filled with 0. In addition to the aforementionedadvantage, this exemplified method enables to obtain a selectivearterial image with high contrast while reducing influence of time shitof three-dimensional measurement.

k-Space data acquisition is not limited to the square matrix as shown inFIGS. 4 and 7 but only data within a circle (ellipse) centering aroundthe origin (number 15) may be acquired as shown in FIG. 8. In FIG. 8,the slice encode number of 5 and the phase encode number of 9 are alsoexemplified but, in this example, a region where both ky and kz becomehigh-frequency component (outer side of the circle) is not sampled anddata on concentric circles centering around the origin (number 15) aresampled. Data acquisition order is, as shown by circled numbers, in sucha manner that the lower half is sampled from a distant point from theorigin toward the origin and the upper half is sampled from the origintoward distant point.

In this case, the same effect as that of the method illustrated in FIG.4 can be obtained. In this case too, data may be compensated usingmeasured data.

The first embodiment of the present invention, in which the k-space isdivided into two regions and sampling points are divided into two groupscorresponding to the region, has bee explained hitherto. The samplingpoints may be divided into two groups regardless of the region.

The second embodiment of the present invention, in which the samplingpoints of the k-space are divided into two groups in such a manner thatsampling points having a relationship of complex conjugate belongdifferent groups.

In this embodiment, when matrix points of sampling points of thek-space, e.g. ky-kz space, are to be divided into two, the followingconditions should be satisfied: 1) the two groups share the origin andthe sampling points of the two groups are in a relation of complexconjugates, 2) adjacent sampling points in the k-space belong todifferent groups. Then these two groups are sampled successively.

As a simplified example of data-acquisition method according to thesecond embodiment, FIG. 9 shows k-space having a matrix of 8*8 with aslice encode number of 8 and a phase encode number of 8. This matrix hassixty four of matrix points (sampling points), which are divided intothe first group shown in the right side of the figure and the secondgroup shown in the left side. The matrix points belonging to the twogroups are in a relation of complex conjugate and the adjacent pointsbelong to the different group. However, adjacent points near the originhave to belong to one group in order to satisfy the relationship ofcomplex conjugate.

Sampling of the first group of two, which is sampled first, is startedwith a distant point from the origin 0 and is controlled from thehigh-frequency component to the low-frequency component such that thedistance from the origin 0 to a sampling point becomes closerprogressively. Sampling of the second group is controlled in an oppositemanner from the low-frequency component to the high-frequency componentsuch that the origin 0 or the neighborhood is sampled first and thedistance from the origin 0 to a sampling point becomes more distantprogressively.

The circled numbers in the figure show the data acquisition order.Sampling points having the same number have no priority and either maybe sampled first.

In the first group to be sampled first, measurement is started with themost distant matrix point (number 1) from the origin (number 33 isassigned), i.e., the highest-frequency component and proceeded to thematrix point of number 2 next, the matrix point of number 3 and so on innumerical order. Measurement of the second group is succeeded, in whichthe nearest point (number 34) from the origin, the low-frequencycomponent, is sampled first and the distance from the origin to a samplepoint increases progressively.

An example of a contrast MRA by the above MRI apparatus employing such adata-acquisition method now will be explained with reference to FIG. 10.

In the same manner as that of the first embodiment, an object to beexamined is placed in a measurement space within the static magnet fieldmagnet and a measurement region including a blood vessel of interest isdetermined. Then a timing measurement is carried out. In this case too,the timing measurement is performed using the test injection method forexample. Specifically, a small amount of a contrast agent (about 1-2 ml)is injected into the object and a time-signal curve in the targetportion is obtained as shown in FIG. 10. Arrival time t1 of the contrast(a time of signal intensity peak) is found from the curve and timing ofa main measurement is determined based on the result.

After the timing measurement, the main measurement is performed as shownin FIG. 10(b). The main measurement may be carried out only afterinjection of the contrast agent but preferably carried out before andafter injection. The measurements are carried out successively beforeand after injection for the same slice or slab position under the samecondition.

The imaging sequence is a short TR sequence based on thethree-dimensional gradient echo method as shown in FIG. 3. In this case,since an imaging object is blood flow, a gradient magnetic field forrephasing a dephase caused by flow, i.e., gradient moment nulling may beadded to the sequence. However, this is not essential and a simplegradient echo is rather preferable in order to shorten the TR/TE.

Once the repetition time TR of the pulse sequence, a matrix size (sliceencode number and phase encode number) and a number of addition aredetermined, the measurement time T is determined. Then starting time t2(a time from injection of a contrast agent to the beginning ofmeasurement) is determined based on the arriving time t1 at a bloodvessel of interest found in the aforementioned timing measurement suchthat a measurement of data in a low-frequency region of the ky-kz spaceare sampled at a time when the contrast agent arrives at the bloodvessel.

First, a measurement of the first group is started and then ameasurement of the second group is performed. In this case, thesequencer 4 controls both a slice direction gradient magnetic fieldpulse and a phase encode direction gradient magnetic field pulse so asto sample from a high-frequency component toward a low-frequencycomponent sequentially in a region measured first, and to sample from alow-frequency component to a high-frequency component progressively inthe region measured thereafter. Thus three-dimensional image data areobtained.

In the data-acquisition method illustrated in FIGS. 9 and 10, all of thesampling points are sampled for both of the first and second groups.However, as shown in FIG. 10(c), another data-acquisition method may beemployed in which sampling of a predetermined high-frequency componentis omitted and a low-frequency component is sampled for a short time.

An example of such a data-acquisition method is shown in FIG. 11. In thefigure, the k-space having matrix size of 8*8, a slice encode number of8 and phase encode number of 8, is illustrated. Although conditions ofdividing k-space into two groups in this example is the same as those ofthe example shown in FIG. 9, a predetermined high-frequency component isnot sampled and only low-frequency component is sampled in the firstgroup which is measured first. In the illustrated method, only matrixpoints on the 4*4 matrix among the matrix points of the k-space aresampled. Among these matrix points in the low-frequency region, a mostdistant matrix points from the origin (point of number 9) is sampledfirst, and sampling is proceeded to a point of number 2 next, a point ofnumber 3 third and to the origin.

In the second group, similarly to the example shown in FIG. 9, samplingis started with a matrix point (number 10) adjacent to the origin andproceeded toward the most high-frequency component in an order accordingto the distance from the origin, and thus overall points belonging tothe second group is sampled.

In this case, high-frequency data which have not sampled in the firstgroup are estimated from data based on the complex conjugaterelationship between the first group and the second group. A manner ofestimating the data which have not been sampled may be a known methodbased on a half-Fourier transformation. The procedure of the estimationis shown in FIG. 12. Sampled data are subjected to one-dimensionalFourier transform in a frequency encode direction (kx direction) toproduce a three-dimensional hybrid space sampled data, from whichthree-dimensional data is estimated. Then the estimated hybrid spacedata and combined with the sampled data to obtain a complete hybridspace data. Fourier transform of this complete hybrid space dataproduces three-dimensional image data. Employing the estimation of dataprevents degradation of special resolution even if the number of datapoints is reduced.

In this embodiment, similarly to the data-acquisition method shown inFIG. 9, the low-frequency component of k-space is sampled when thecontrast agent arrives at the blood vessel of interest and the signalintensity of the blood vessel is enhanced most. Accordingly, images ofan artery can be imaged with high contrast.

In addition, since the concentration of a contrast agent (signalintensity) rapidly increases after injection of the contrast agent asshown in FIG. 12, if this imaging method is performed so that a samplingtime of the most low-frequency component is synchronized with a signalintensity peak, sampling of unnecessary data before arrival of thecontrast agent is omitted because of its short measurement time beforethe peak.

Results of simulation of artery-vein separation using differentdata-acquisition methods are shown in FIGS. 13 and 14. The simulationwas conducted using an imitated artery and vein, through which acontrast agent is passed at a flow speed of 40 cm/s, a circulating timebetween the artery and vein of 7 seconds, and an injecting speed of 2cc/sm, and images of the artery and the vein were produced usingdifferent imaging methods. FIG. 13 shows signal intensities obtainedunder the above conditions. As shown in the figure, a peak intensity ofsignals from the artery is observed first and then a peak intensity ofsignals from the vein appears behind. FIG. 4(a) shows an image producedby the imaging method of the present invention and (b) shows an imageproduced by the elliptical centric ordering.

As understood from the figure, not only the artery but also the vein areimaged and these two blood vessels are not completely separated by theelliptical centric ordering. On the other hand, only the artery can beimaged with high contrast in the imaging method of the presentinvention.

Although a gradient echo sequence has been also exemplified as athree-dimensional MRA sequence in the second embodiment, an EPI (EchoPlaner Imaging) sequence which enables to sample a plurality of echosignals with one excitation or a multi-shot EPI may be also employed.

Further, the k-space data to be acquired are not limited to a squarematrix data as shown in FIGS. 9 and 11 may be data within a circle(ellipse) centering around the origin as shown in FIG. 15. In thefigure, the first group, which is indicated by a real line, is sampledfrom the high-frequency component toward the low-frequency componentsuch that a distant point from the k-space origin is sampled first andthe distance to the origin decreases. Contrarily, the second group issampled from the low-frequency component toward the high-frequencycomponent such that sampling is started with the k-space origin orneighborhood and the distance from the origin to a sampling pointincreases. The same effect as that of the method illustrated in FIGS. 9and 11 can be obtained and sampling of the high-frequency component ofthe first group can be omitted.

INDUSTRIAL APPLICABILITY

As explained hitherto, the MRI apparatus of the present inventionemploys a sampling control method in which ky-kz space matrix points(sampling points) are divided into two groups and the first group to besampled first is sampled from the high-frequency component toward thelow-frequency component such that the distance of a sampling point tothe k-space origin progressively decreases and the second group issampled from the low-frequency component toward the high-frequencycomponent such that the distance of a sampling point to the k-spaceorigin progressively increases, thereby reduces influence of imagingtiming error and provides high contrast images in which a goodartery-vein separation is achieved. In addition, by reducing a number ofsampling points of one of the groups, a measurement time can beshortened.

What is claimed is:
 1. A magnetic resonance imaging apparatus comprisingstatic magnet field generating means for generating a static magnetfield in a space where an object to be examined is placed, gradientmagnetic field generating means generating gradient magnetic fields inthe space in a slice direction, phase encode direction and readoutdirection, transmitting means for applying RF magnetic field to causenuclear magnetic resonance in atomic nuclei of a living tissue of theobject, receiving means for detecting echo signals emitted by thenuclear magnetic resonance, control means for controlling the gradientmagnetic field generating means, transmitting means and receiving means,signal processing means for performing image reconstruction using theecho signals detected by the receiving means, and display means fordisplaying the produced images, wherein the control means performs athree-dimensional sequence including a slice encoding step and a phaseencoding step and, upon performing the sequence, divides sampling pointsof k-space defined by a slice encode number and phase encode number intotwo groups and controls the gradient magnetic field generating means ofthe slice direction and the phase encode direction such that a distancefrom the k-space origin to a sampling point progressively decreases inthe measurement of the first group and a distance from the k-spaceorigin to a sampling point progressively increases in the measurement ofthe second group.
 2. The magnetic resonance imaging apparatus of claim1, wherein the two groups respectively belong to either of two regionsdivided from the k-space.
 3. The magnetic resonance imaging apparatus ofclaim 1, wherein the two groups share the k-space origin and are in arelation of complex conjugate.
 4. The magnetic resonance imagingapparatus of claim 3, wherein the sampling points of the k-space aredivided such that adjacent sampling points belong to different groups.5. The magnetic resonance imaging apparatus of claim 1, wherein at leastone of the groups includes points which are not sampled.
 6. The magneticresonance imaging apparatus of claim 1, wherein an addition set of thetwo groups is within a circle inscribed in the k-space.
 7. A dataacquisition method of acquiring three-dimensional image data byrepeating a plurality of times a step comprising selective excitation ofa predetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions andcollection of echo signals emitted from the region while changing theintensities of the gradient magnetic fields, wherein sampling points ina measurement space defined by the intensities of the two directionencoding gradient magnetic fields are divided into the first and secondgroups, the first and second groups are measured successively, in themeasurement, the first group to be measured first is sampled such thatthe distance from the measurement space origin to a sampling pointdecreases in a sampling order and the second group to be measured afteris sampled such that the distance from the measurement space origin to asampling point increases in a sampling order.
 8. A data acquisitionmethod of acquiring three-dimensional image data by repeating aplurality of times a step comprising selective excitation of apredetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions andcollection of echo signals emitted from the region while changing theintensities of the gradient magnetic fields, wherein sampling points ina measurement space defined by the intensities of the two directionencoding gradient magnetic fields are divided into two, the dividedregions are measured successively, in the measurement, the first regionto be measured first is sampled such that the distance from themeasurement space origin to a sampling point decreases in a samplingorder and the second region to be measured after is sampled such thatthe distance from the measurement space origin to a sampling pointincreases in a sampling order.
 9. A data acquisition method of acquiringthree-dimensional image data by repeating a plurality of times a stepcomprising selective excitation of a predetermined region of an objectto be examined, application of encoding gradient magnetic fields atleast in two directions and collection of echo signals emitted from theregion while changing the intensities of the gradient magnetic fields,wherein sampling points in a measurement space defined by theintensities of the two direction encoding gradient magnetic fields aredivided into the first and second groups such that the groups share theorigin and are in a relation of complex conjugate, and adjacent matrixpoints belong to different groups, the first and second groups aremeasured successively, in the measurement, the first group to bemeasured first is sampled such that the distance from the measurementspace origin to a sampling point decreases in a sampling order and thesecond group to be measured after is sampled such that the distance fromthe measurement space origin to a sampling point increases in a samplingorder.
 10. A data acquisition method of claim 7, wherein a part of thesampling points is sampled in one of the two groups and all of thesampling points are sampled in the other group.
 11. A three-dimensionalangiography using a contrast agent comprising the steps of measuring atime from injection of the contrast agent till arrival of the contrastagent at a blood vessel of interest, and performing a data acquisitionaccording to a method of any one of claims 7-9, wherein a time of thebeginning of the measurement of the first group is controlled such thatthe end of the measurement of the first group is coincident with anarrival time of the contrast agent.